1. Technical Field
This invention relates to an organic electroluminescent (EL) device, and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
2. Background Art
Organic EL devices have enabled to form a device of large area on a glass, and considerable research and development on such organic EL devices have gone on for their use as a display. In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of ITO etc., a hole transporting layer of an organic amine compound, an organic light emitting layer of a material exhibiting electronic conductivity and intense light emission such as Alq3, and an electrode of a low work function metal such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. The organic compound layers are typically of two- or three-layer structure.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 3 and 4 illustrate typical configurations of organic EL devices.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Attempts have been made to improve the luminous efficiency of these organic EL devices. With the prior art device configuration, however, for reasons of poor hole blocking of the electron injecting and transporting layer, it was difficult to achieve efficient recombination of electrons with holes in the light emitting layer and hence, to provide a device with a fully satisfactory efficiency.